kramacha1979 wrote:
In B why can't we say than they have been <set>
no.
you cannot omit a word unless it ALREADY has a complement, used IN EXACTLY THE SAME WAY, earlier in the sentence.your <set>, which you're trying to omit, is a past participle (like "taken" or "eaten").
there is no existing use of "set" as such a participle, so you can't omit this one.
--
example of a sentence in which you
can make this sort of omission:
the temperature can't be set until the pressure has been <set>.in this case, the existing "set" is a past participle, as is the omitted "set", so we're all good.